Eggshell – formation, structure and meaning
Egg shell is formed in the hen’s biomineralisation process. This process involves combining the hydrated calcium ions Ca2+ with the bicarbonate anhydrate HCO3– formed in the presence of carbonate anhydriosis.
2 CO2 + H2O ß——à 2 HCO3–
Ca2+ + 2 HCO3– ß——à Ca(HCO3)2
The calcium carbonate precipitating from the liquid environment has a porous structure and contains a small amount of organic substances (about 5%) . Among the organic substances are proteins (about 1%)  and significant amounts of fatty acids (2% – 4%) in the form of lipids and lipoproteins . The basic dye of the shells is the pigment biliverdin .
The main task of the egg shell is to protect the developing embryo against harmful external factors. In addition, the porous shell structure allows gas exchange. Skorupka is the source of a number of cations, mainly calcium. As a natural inorganic material, additionally reinforced with organic scaffolding, it is relatively durable.
Sources of calcium carbonate origin
A common source of calcium ion are synthetic and fossil forms of calcium carbonate (aragonite and calcite). The very high content of Ca2+ ions, the low cost of production and the relative high bioavailability (ie the degree of absorption by the human or animal body) is decisive.
Much more than natural mineral biogenic calcium carbonate is valued more than natural minerals. Biogenic calcium carbonate comes from, among others from shells of marine crustaceans and external skeletons of marine animals (including non-precious varieties of white coral), or just from shells of chicken eggs [note. Małgorzata Wiewióra].
In the case of biogenic calcium carbonate, characteristics such as its porosity (facilitating absorption) are important, as well as the presence of trace amounts of other elements (eg strontium and fluorine), beneficial for the metabolism of living organisms. The origin of natural calcium carbonate is important.
Hazards can be caused by low amounts of heavy metals. The main threat may be the presence of lead, aluminum, cadmium or mercury  – this is why the material should be obtained from reliable sources and from chickens using fully balanced food [note. Wojciech Wiewióra].
Chemical composition of eggshells
On the basis of the studies by Schaafsma et al. , it is believed that eggshells after hatching contain similar contents of calcium ions as synthetic calcium carbonate. In the case of egg shells, there are more [health benefits – WW] strontium and selenium. The reduced amount of harmful heavy metals in the shells of chicken eggs such as vanadium, chromium, lead, aluminum and cadmium may be caused by the impaired its bioavailability in the mother’s body. Among the trace amounts of other elements, the presence of selenium, zinc, copper and strontium is essential , . It is believed that strontium has an inhibitory effect on osteoclasts and stimulates the activity of osteoblasts (bone forming cells).
According to Kusumi et al.  porous calcium carbonate originating from chicken egg shells is much more easily dissolved (digested) than its synthetic varieties and can more effectively improve bone mineral density , , , . In addition, the antibacterial activity of proteins present in the shells of chicken eggs was observed , . Of course, the possibility of multiplying harmful bacteria in case of defective storage of the shells should be avoided.
The role of calcium carbonate in deacidification of soils
Traditional methods of soil deacidification are based on the use of ground limestone, i.e. a relatively hard and hard-to-absorb stone limestone to neutralize organic acids. In addition, fossil calcium carbonate is often contaminated with significant amounts of magnesium, iron and manganese salts.
Own research and discussion of results 
In order to confirm the structure of egg shells, a number of analyzes were performed using advanced instrumental methods. The tests carried out were aimed at confirming the chemical structure of chicken egg shells and simple methods of instrumental analysis.
In the first place, the presence of calcium ions in the examined shells was demonstrated by means of X-rays.
In addition, on the basis of infrared studies (IR spectra in KBr pellets in the range of 400 to 4000 cm-1), the presence of calcium carbonate was demonstrated. It can be noticed the stretching and deformation vibrations of a permanent form of crystalline calcium carbonate – calcite . Similar IR spectra are observed in the case of white coral . The wide band at approx. 3400 cm-1 comes from a small amount of moisture in the sample.
The spectra made in the form of KBr pellets are characterized by relatively low noise and well-formed peaks. The use of much less labor-consuming in terms of sample preparation for testing the internal rebound technique (ATR) allows to obtain significantly lower quality spectra. It should be noted, however, that such spectra also contain all relevant information allowing for the presence of calcium carbonate in the test shells. Similar results can be obtained by a technique complementary to IR spectroscopy, i.e. by means of Raman scattering . In this case, the bands are much narrower and the spectrum is easier to analyze.
Evaluation of two egg shell samples for deacidification and its enrichment in selected elements 
Two samples of crusted egg shells (dried, sterilized and ground product) were provided:
- coarse (0.5-2 mm)
- fine-grained (less than 0.05 mm).
On the basis of literature and own research, it can be concluded that the formulations provided contain calcium carbonate (calcite as a more stable crystal form).
Due to the significant fragmentation and the possibility of preparing aqueous suspensions, it was found that such preparations in the form of a dry or aqueous suspension are perfect for de-acidification of the soil and its enrichment in a number of beneficial trace elements improving plant growth.
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